The present invention relates to a bulk high-temperature superconductive magnet with freely variable strength of magnetic field obtainable by combining a high-temperature superconductive bulk form with high critical current with an normal conductive or superconductive coil, which allows the stabilization of conventional superconductive coil and the more extended application of superconductive magnet. The magnet with this structure is utilizable for, for example, the stabilization of superconductive coil for magnetic levitated train, etc.
With the discovery of oxide superconductor represented by R-Ba-Cu-O type (R denotes rare-earth elements, hereinafter it means the same) with critical temperature (T.sub.c) exceeding 90K, it has become possible to use liquid nitrogen as a coolant for superconductor. For putting the superconductor into practice, it is required to process this into wire, tape or the like. It is the status quo however that, in this form, the critical current being most important in the practice of superconductor is low and has not reached the practical level at 77K.
For example, Bi-Sr-Ca-Cu-O type superconductor is relatively easy to process into tape. Thus, a tape with length exceeding 100 m has already been made and a pancake coil that generates a magnetic field exceeding 1 T is manufactured, but it exhibits only around 0.1 T at a temperature of liquid nitrogen at most.
In the case of Bi-Sr-Ca-Cu-O type material, the anisotropy is significant in the crystal structure and, while the critical current is relatively high when applying the magnetic field perpendicularly to the c-axis of crystal, it becomes very low when applying parallel, which is considered to be a problematic point at the time of using liquid nitrogen.
On the other hand, with R-Ba-Cu-O type superconductor made by melt process, the optimization of flux pinning effect has been achieved though in the state of bulk, and very high critical current at practical level is achieved even at a temperature of liquid nitrogen. Such bulk form exhibits a high repulsive force and attractive force through the interaction with magnetic field, hence application to bearing etc. is investigated. Moreover, trapping of magnetic field is also possible, leading to a magnetic field exceeding 1 T at a temperature of liquid nitrogen.
With conventional superconductor, if attempting to use it in bulk form, so-called quenching phenomenon, in which the superconductivity is broken abruptly by a small external disturbance, occurred because of low specific heat, making it impossible to utilize in the stable state. In the case of linear motor car, the practicality being investigated currently, this quenching is posing a problem. Whereas, the high-temperature superconductor has an advantage of being usable stably even in bulk.
As described above, there is a problem that, with bulk superconductor alone, high magnetic field can be generated, but the control of that generated magnetic field is difficult. Moreover, there is a problem that, with superconductive coil alone using a tape of high-temperature superconductor, the generating magnetic field can be controlled by the level of current, but the generating magnetic field is too weak at a temperature as high as that of liquid nitrogen.
In addition, while the superconductive coal capable of generating large magnetic field is manufactured using low-temperature superconductive material and the application to linear motor car is investigated utilizing the mutual repulsion between magnets, it cannot necessarily be said that the practicality is high.
As a result of extensive investigations for overcoming the respective drawbacks of high-temperature superconductive bulk magnet, high-temperature superconductive coil and low-temperature superconductive coil, the inventors have known that a composite constituted by appropriately combining high-temperature superconductive bulk form with normal conductive or superconductive coil is possible to be utilized in the stable state, leading to the completion of the invention.